In an optical communication system, an optical transmitter may encode digital information in the form of bits by mapping the bits to symbols, and then using a particular modulation scheme to modulate one or more optical carriers with the symbols. The optical transmitter thereby generates an optical signal to be transmitted via an optical communication channel to an optical receiver, where the optical signal is representative of the digital information. The optical receiver may process an optical signal received via the optical communication channel to recover estimates of the symbols, estimates of the bits, or both.
The optical signal received at the optical receiver may comprise a degraded version of the optical signal that was generated at the optical transmitter. Various components of the optical communication system that may contribute to signal degradation include optical fibers, optical amplifiers, filters, isolators, and the like. Amplifier noise, optical nonlinearity, polarization-dependent loss or gain (PDL or PDG), polarization mode dispersion (PMD), frequency-dependent loss, and other effects may introduce noise and/or distortion into the signal. The amplitude of the noise relative to the amplitude of the optical signal may be characterized by signal-to-noise ratio (SNR), or alternatively by noise-to-signal ratio (NSR). The NSR may be convenient when dissecting noise sources. A high NSR may result in noisy symbol estimates, which may in turn produce erroneous estimates of the bits. The probability that bit estimates recovered at the optical receiver differ from the original bits encoded at the optical transmitter may be characterized by the Bit Error Ratio or Bit Error Rate (BER). A given application may have a maximum BER tolerance. For example, an application may require that the BER does not exceed 10−15.
Forward Error Correction (FEC) techniques may be used to reduce the BER. Instead of mapping the original bits of information from the client (referred to as client bits) directly to symbols, the client bits may first undergo FEC encoding based on a chosen FEC scheme. The resulting FEC-encoded bits include redundant information, such as parity or check bits. The bit estimates recovered at the optical receiver are estimates of the FEC-encoded bits that were generated at the optical transmitter. These estimates may undergo FEC decoding at the optical receiver based on the chosen FEC scheme. The FEC decoding makes use of the redundant information that was included in the FEC-encoded bits in order to detect and correct bit errors. Ultimately, estimates of the original client bits may be recovered from the FEC-decoded bit estimates.
FEC encoding is advantageous in that it acts to reduce the received BER without the need to resend data packets. However, this is at the cost of an increased overhead. The amount of overhead or redundancy added by FEC encoding may be characterized by the information rate R, where R is defined as the ratio of the length of the input data sequence to the length of the output data sequence after FEC encoding (which includes the overhead). For example, if FEC encoding adds 25% overhead, then for every four bits that are to be FEC-encoded, the FEC encoding will add 1 bit of overhead, resulting in 5 FEC-encoded bits to be transmitted to the optical receiver. This corresponds to an information rate R=4/5=0.8.